Except for the Hagfish, Lamprey, and Their Close Relatives, All

Except for the hagfish, lamprey, and their close relatives, all vertebrates have paired jaws; how did jaws form in an evolutionary clade that already had a skeleton?

A) A mutation causing extra skeletal cell division allowed the extra cells to form the jaws.
B) A modification of an existing skeletal element occurred; in this case the gill arch was modified.
C) Reproduction between a nonjawed chordate and a jawed arthropod produced jawed offspring.
D) To date biologists have been unsuccessful in explaining the origin of jaws.